The present invention relates to an auto-tracking method and apparatus of a magnetic recording/reproducing apparatus of the helical scan system and, more particularly, to an auto-tracking method and apparatus of the type such that the running phase of a magnetic tape is controlled so that the envelope level of the reproduced signal becomes maximum and thereby allowing a video head to scan a track on the magnetic tape.
Hitherto, in home-use magnetic recording/reproducing apparatuses (hereinafter, referred to as VTR) of the helical scan system, the tracking control is performed so that the video head accurately scans the track on the magnetic tape. As one of such tracking controls, there has been conventionally known a system in which the rotational phase of the capstan motor (accordingly, the running phase of the magnetic tape) is controlled such that the phase difference between the control signal reproduced from the magnetic tape and the reference signal generated in the VTR becomes a predetermined value.
On the other hand, conventional VTRs are generally constituted such that the reproducing modes in which the running speeds of the magnetic tape differ from each other can be selected, and ordinarily, the standard reproducing mode and the one-third-speed reproducing mode in which the running speed of the magnetic tape is slower than that in the standard reproducing mode can be selected.
However, in such VTRs, dedicated video heads are provided for each of those reproducing modes and also these dedicated video heads are arranged on the rotary cylinder at the positions which are deviated from one another. Thus, the relations between the rotational phases of the video heads in the best tracking state (namely, the state whereby the video heads are accurately scanning the tracks) and the phases of the control signals in those reproducing modes differ. In general, the rotational phase of the cylinder motor to rotate and drive the video heads is controlled by the reference signal which is common in the foregoing two reproducing modes, so that the rotational phase on the side of the video heads cannot be changed. Therefore, by controlling the reference signal to control the rotational phase of the capstan motor, namely, the running phase of the tape has to be made different between the above-mentioned respective reproducing modes.
Therefore, in such VTR, there is needed an auto-tracking apparatus which can also automatically control the phase of the reference signal to control the rotational phase of the capstan motor upon starting of the reproducing mode or in association with the switching between the standard reproducing mode and the one-third-speed reproducing mode.
As an example of such an auto-tracking apparatus, there has been known an auto-tracking apparatus in which a tracking control signal is detected using the envelope level of the reproduced signal and by using this control signal, the running phase of the magnetic tape is controlled so that the foregoing envelope level becomes maximum.
This auto-tracking apparatus uses the principle that, as shown in FIG. 1, the envelope level of the reproduced signal (hereinafter, referred to as a reproduction envelope level) generally changes with the change in the position of the video head in the direction of the width of the track on the magnetic tape (hereinbelow, this position is referred to as a tracking phase; it is assumed that when the central line of the video head coincides with the central line in the longitudinal direction of the track, the tracking phase is zero and that when the position of the video head is deviated by the track pitch (namely, the distance between the central lines of the adjacent tracks), the tracking phase is 2 .pi.). This reproduction envelope level changes like a sine wave in such a manner that it becomes maximum when the tracking phase is 2n .pi. and minimum when the tracking phase is (2n-1) .pi., wherein n is 0, 1, 2, . . . Therefore, if this reproduction envelope level is detected and the running phase of the tape is controlled in accordance with this level to change the tracking phase, the reproduction envelope level can be made maximum.
In the above-mentioned auto-tracking apparatus, the reproduction envelope level is periodically sampled and when there is a difference between the sampling value and the sampling value which has been detected immediately before, the running phase of the magnetic tape is changed such that the video head is moved by a predetermined amount in the direction of the width of the track, thereby changing the tracking phase. As shown in FIGS. 2(a) and 2(b), when the tracking phase is sequentially changed and the envelope level again increases, there is a tendency such that the sampling value also sequentially increases. The sampling value changes from the increasing state to the decreasing state at the turning point where the reproduction envelope level becomes maximum. That is, the auto-tracking apparatus automatically searches the tracking phase at which the sampling value changes from the increasing state to the decreasing state.
On the other hand, in such a conventional auto-tracking apparatus, to improve the accuracy in automatic search of the tracking phase, it is necessary to make small the change amount per one time of the running phase of the magnetic tape and thereby changing the tracking phase little by little. However in the case where the tracking phase is automatically searched by changing the tracking phase little by little as described above, the number of changes of the tracking phase until the maximum reproduction envelope level is derived, namely, the number of steps becomes extremely large, so that it takes long time to obtain the optimum tracking state. Particularly, in FIG. 1, the longest search time is necessary in case of searching from point A where the tracking phase at the start of the search is near 0.degree. in the direction indicated by an arrow. As described above, there is a drawback such that when the tracking search time is long, it takes a long time (monitor cut time) from the start of the playback until the stable reproduced image is derived.
In addition, in the above-mentioned method known as an edge detecting method by which an envelope phase at which the envelope level starts changing from increasing to decreasing is searched, the following fatal problems are caused.
First, actually, the reproduction envelope level to the tracking phase does not accurately become like a sine wave as shown in FIG. 1 but has a small peak at the position where the tracking phase is near (2n-1) .pi. as shown in FIG. 3. This is because the noise is obtained from the video head in the worst tracking state wherein the video head is largely deviated from the track. If the peak occurs in this way, the reproduction envelope level obviously changes from the increasing state to the decreasing state, so that the apparatus erroneously recognizes that this wrong state is the best tracking state and this state is locked, and the state in which no playback image is obtained is held.
Second, the relation between the tracking phase and the reproduction envelope level is not determined on the one-to-one correspondence basis as shown in FIG. 1 or 3. When the reproduction envelope level is detected while making the tracking phase constant, this reproduction envelope level largely fluctuates. This fluctuation is caused due to a variation in contact state between the video head and the magnetic tape, or the like. As the result of the actual measurement, as shown in FIG. 4, the reproduction envelope level has a variation width indicated by the hatched portion. Therefore, in the case where the tracking phase is changed such that t.sub.1 .fwdarw.t.sub.2 .fwdarw.t.sub.3, if the reproduction envelope level simply exhibits like a sine wave as shown in FIG. 1, the reproduction envelope level is obviously expected to be monotonously reduced; however, the reproduction envelope level changes such that c.sub.1 .fwdarw.c.sub.2 .fwdarw.c.sub.3 and the level c.sub.1 becomes the peak level, so that it is determined that the tracking state at the tracking phase of t.sub.2 is the best state. On the other hand, even in case of changing the tracking phase such that t.sub.4 .fwdarw.t.sub.5 .fwdarw.t.sub.6 as well, in spite of that the reproduction envelope level should monotonously increase, it becomes maximum at c.sub.5 and it is decided that the tracking state at the tracking phase of t.sub.5 is the optimum state.
As described above, the auto-tracking apparatus according to a conventional edge detecting method has drawbacks such that the search time becomes long and a malfunction occurs.
One example of a similar auto-tracking apparatus to the above-mentioned conventional auto-tracking apparatus is disclosed in Japanese Patent Unexamined Publication No. 53-90910.